Electron beam recording system and apparatus



Feb. 24, 1970 l E. w. REED, .1R

ELECTRON BEAM RECORDING SYSTEM AND APPARATUS original Filed Nov. s, -1965 3 Sheets-Sheet 2 R w MW I||I|| l I l l i I I I .Il s 0 Y M W Tilll--- p lill L l E l-||||||1||- Il. 1 1|. 1-- I l n n 1 1 n l l l l l l l- QJA.

Feb. 24, 1970 E. w. REED, JR l 3,497,762

ELECTRON' BEAM RECORDING SYSTEM AND APPARATUS original Filed Nov. s, 1965 3 Sheets-Sheet 3 fon/ARD I NVE N TOR. MPa-@Je na/VFY;

United States Patent O 3,497,762 ELECTRON BEAM RECORDHNG SYSTEM AND APPARATUS Edward W. Reed, Jr., St. Paul, Minn., assignor to Minne- Sota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Continuation of application Ser. No. 506,188, Nov. 3, 1965. This application Dec. 12, 1968, Ser. No. 800,307 Int. Cl. H011' 29/ 70 U.S. Cl. 315-21 18 Claims ABSTRACT OF THE DISCLOSURE An electron gun apparatus which is capable of blanking an electron beam and adapted for use as a monitoring system and as a servo control system for regulating beam current is shown wherein the apparatus includes an electron beam generating means, a scanning means which includes a iirst deflection means for scanning the beam in a scan pattern on a target, electron beam collecting means located outside but adjacent the electron path and positioned stationary between the electron beam generating means and the iirst deflecting means and a second deflecting means positioned between the electron beam generating means and electron beam collecting means for deflecting the beam from the target. In one embodiment, the electron beam is deflected into an electron beam collecting means which is a Faraday cage.

-man This application is a continuation of Ser. No. 506,188, filed Nov. 3, 1965, and now abandoned.

This invention relates to improvements in electron gun apparatus.

In one aspect, this invention is directed to an electron gun apparatus of the type adapted to produce an electron beam wherein there is located internally within the electron gun a combination of elements which are especially adapted to deect an electron beam from a target into an electron beam collector during selected intervals of electron beam operation.

In another aspect, this invention is directed to a beam blanking system suitable for use with such an electron gun apparatus.

In still another aspect, this invention is directed to a beam monitor system suitable for use with such an electron gun apparatus.

In another aspect, this invention is directed to a an electron gun apparatus.

In the following brief discussion, the prior art is considered for each aspect of the invention:

To generate an electron beam, a typical electron gun apparatus employs an electron beam generating means comprising a cathode, grid(s) and anode(s), all generally axially aligned with respect to one another by appropriate mounting means, enclosed in an evacuatable housing. The present invention provides a new, useful and improved electron gun construction containing a coacting combination of an electron beam collection means and a means for deecting said electron beam from a target into the beam collection means. Additionally, the electron gun apparatus usually includes an electron beam scan deflecting means for deflecting said electron beam in a scan pattern on said target.

Applications involving the use of electron gun apparatus usually require that the electron beam be extinguished or blanked relative to a target for a predetermined period of time in a controlled manner. For example, conventional television devices employ, in combination with electron gun apparatus, electrostatic or electromagnetic beam focusing means, and electrostatic or electromagnetic elec- 3,497,762 Patented Feb. 24, 1970 ICC tron beam deecting means located after the anode(s) along the predetermined path followed by the beam in traveling from the electron beam generating means to bombard a target. During operation, it is necessary to prevent the beam from scanning the target during retrace interval, which is the period from the end of one scan line to the beginning of the next scan line. Such prevention, termed blankingj has heretofore been accomplished in commercial electron guns during the beam retrace by (a) placing a negative charge on the gun grid such that electron emission through the grid is cut off; (b) placing a positive charge on the gun cathode such that electron emission through the grid substantially ceases; or (c) defocusing the beam to an extent such that it does not appear to scan the target.

lUse of large negative voltages for blanking have several other attendant disadvantages. The electron beam is blanked by applying a large negative voltage to the video amplifier such that the blanking voltage is superimposed upon the reference level of the video signals. At the end of the blanking interval, the video amplifier returns to a reference level which is usually different from the reference prior to blanking.

Another disadvantage of the known blanking systems is that the continual change of the electron beam current level from a reference level to a cut off blanking level subjects the high voltage regulator to abrupt changes in loading conditions thereby imposing severe high voltage regulating conditions on the regulator.

In certain applications utilizing electron gun apparatus, the target is at ground potential and the grid-cathode assembly is operated at a high negative potential. Thus, control of the grid or cathode to produce blanking requires high negative voltages comparable in magnitude to the high negative operating potential. Generation of such high negative voltages for blanking purposes has proved to be complicated and diflicult.

In other recording systems, a recording material which is sensitive to and exposed by an electron beam is used as the target. If such a recording system utilizes blanking which defocuses the beam, the defocused beam would be capable of exposing the electron sensitive recording material.

The present invention overcomes each of the above disadvantages by incorporating into an electron gun apparatus the coacting combination of a beam collector and means for deecting the beam from the target into the beam collecting means. Blanking of the electron beam produced by the electron gun apparatus is achieved in a simple manner by completely removing the electron beam from the target for a predetermined interval equal to the blanking interval. In the blanking system of the present invention, blanking voltages need not be applied to the video amplifier to disturb its reference level. Abrupt changes in electron beam current level are eliminated and the resultant loading upon the high voltage regulator reduces the voltage regulation requirements thereof.

The prior art practice of indirectly measuring electron beam current in an electron g-un by measurement of currents associated with elements within the beam generator means has been generally unsatisfactory because of certain inaccuracies inherently associated with such techniques. For example, secondary electron emission from the anode and aperture structures will vary causing an erroneous measurement of the beam current.

The present invention provides a beam monitoring system for measurement of electron beam current by deflecting the electron beam, prior to bombarding the target, into a Faraday cage. The beam current is measured during the interval in which the electron beam remains Yin the cage. Typically the interval of deflection is determined so as not to interfere appreciably with the operations being performed with the beam on the target. Blanking times (eg. the times that the beam is removed from the target and residing in the cage) can be as short as several microseconds, depending upon such factors as the deflection means used, the time available for sampling, and the conditions of sampling.

While the electron beam is in the Faraday cage, measurement of the electron beam current can be accomplished simply and directly since the current generated in the cage by the beam is directly susceptible to measurement by conventional voltage and/ or current metering devices.

Typical use situations in which a beam monitoring system of this invention finds application include electron beam recorders, cathode ray tubes, electron beam welders, electron beam milling machines, electron beam evaporators and the like.

A monitoring system of this invention can be used for monitoring an electron beam emanating from a source Ywhich produces either a continuous or a pulsed electron beam. In the event a pulsed electron beam is to be monitored, it will be appreciated that the apparatus to be used for measuring the pulsed electron beam current must be capable of such measurements.

A monitoring system of this invention can be used in combination with a blanking system independently of whether or not the electron gun apparatus includes additional beam focusing means and beam scan deflection means.

In addition to the aforedescribed problems in blanking and monitoring an electron beam, the art has heretofore had only limited ways of dynamically controlling or stabilizing electron beam current. In general, the operating level of the electron beam current is regulated by adjusting the D.C. (direct current) operating level of the electron beam generating means. The operating level can be adjusted by varying the space charge between the grid and cathode in the grid-cathode assembly, or by varying the cathode temperature which controls electron emission from the cathode in the grid-cathode assembly. In the usual case of varying the space charge, the D.C. operating level of the grid determines the magnitude of current associated with an electron beam generated in an electron gun assembly for a given potential difference between grid and cathode.

A commonly-experienced problem associated with electron guns is the drift or change which occurs in the output current over a period of time, particularly when the potential between the grid and cathode remains xed. Such drift or change is attributed to various causes, for example, to dimensional changes in a gun assembly due to temperature changes. Drifting of electron beam current due to such dimensional changes can be compensated for by altering the potential between the cathode and the grid.

In some applications, cathode bias or self bias is employed to compensate for variances in electron beam current due to such dimensional changes. In such applications, a resistor, in parallel with a capacitor, provides negative feedback which varies the D.C. operating level of the grid as a function of the average beam current. However if the beam current is of a low value, the cathode or self bias is impractical, particularly if the D.C. operating level is to be instantaneously changed, to keep the beam current at a desired level.

The level of electron beam current, as it bombards the target, does not bear a linear relationship with cathode current. In the present invention, the true level of electron beam current at the target is obtained by sampling the level of electron beam current with the Faraday cage and instantaneously adjusting the D.C. operating level of the grid if the electron beam current level is unequal to a desired electron beam current level. The sampling circuitry is part of a beam servo control system, and the use of such a system for sampling and instantaneously adjusting the electron beam current level is novel and unique.

Those skilled in the art will comprehend aspects of this invention from a reading of the present specification taken together with the accompanying drawings wherein:

FIGURE 1 is a diagrammatic illustration of an electron gun construction of the present invention;

FIGURE 2 is a perspective view of a Faraday cage suitable for use in the electron gun apparatus of the present invention;

FIGURE 3 is a schematic diagram illustration of one embodiment of a system for effectuating beam blanking using an electron gun construction of this invention;

FIGURE 4 is a schematic diagram illustrating one embodiment of a system for beam control using an electron gun construction of this invention; and

FIGURE 5 diagrammatically illustrates various waveforms associated with the operation of the beam servo control system of FIGURE 4.

Aspects of the invention are now described and illustrated by reference to specific embodiments:

Referring to FIGURE 1, there is seen a diagrammatic representation in the nature of a vertical sectional view through an electron gun apparatus of the invention, herein designated in its entirety by the numeral 10. Electron gun apparatus 10 employs an electron beam generating means herein designated in its entirety by the numeral 12, and a coacting combination of an electron beam collecting means and a means for detlecting the beam from the target is designated in its entirety by the numeral 14.

A triode type electron gun designed in accordance with the parameters set forth in a book entitled Electron Optics by O. Klemperer published in 1953 by Cambridge University Press is illustrative of an electron gun apparatus for use with this invention.

In the embodiment of FIGURE l, an electron beam is generated as follows. Electrons are emitted from a hot filament cathode 20 and are accelerated towards an anode 22 due to the potential difference existing therebetween. Inter-posed between the anode 22 and the filament cathode 20 is a grid 24 which, operating in conjunction With filament cathode 20, controls the net number of electrons leaving the filament cathode 20. The anode 22 is grounded and the filament cathode 20 and grid 24 are maintained at a high negative potential with respect to the anode 22, for example, a negative potential of about 18,000 volts, by a regulated power supply (not shown).

Optionally, an aperture disk 28 having a limiting aperture 30 centrally formed therein is positioned in the electron gun apparatus 10 so that its aperture 30 is generally coaxial with the gun axis 32. As the accelerated electron beam leaves the anode 22, the electron beam strikes the aperture disk 28. Limiting aperture 30 then limits the diameter of the electron beam to some predetermined maximum diameter. Aperture disk 28 tends to reduce aberrations in beam spot during scanning.

As the electron beam travels along a predetermined path down the gun axis 32, the beam passes through an electromagnetic main focusing lens 34. The main focusing lens 34 functions to focus the electron beam into a Small cross-sectional spot size on the target, which may be the surface of an electron beam sensitive recording medium 40 upon which video information is to be recorded.

Optionally, a second electromagnetic focusing lens, illustrated generally by the dotted line 42, can be used to provide dynamic focusing to compensate for deflection defocusing caused by an electron beam deflection means such as, for example, an electromagnetic deflection yoke 44. Conveniently, a second focusing lens 42 would be positioned adjacent the main focusing lens 34 on that side thereof facing anode 22. This optional second focusing lens is essentially weak and provides only small corrections in the beam focus.

The power supply used to operate the main focusing lens 34 is typically a constant current, low voltage power supply. The power supply used to excite the optional second lens 42 is variable but synchronized with scan rates so that focus is maintained over the entire scan pattern.

While electromagnetic focusing lens means 34 and 42 are employed in the embodiment shown in FIGURE l, it will be appreciated by those skilled in the art that electrostatic focusing lens means can also be used equivalently.

After leaving the focusing lens 34, the focused electron beam passes through the electromagnetic deflection yoke 44. The deflection yoke `44 is adapted to deflect the beam in a scan pattern on the target area in synchronism with the signal to be recorded. Means for obtaining such synchronization are well known, for example in commercial electron gun apparatus, and consequently are not described in detail herein particularly since they form no part of the present invention.

In place of electromagnetic deflection yoke 44, one can employ electrostatic deflection plates as those skilled in the art will appreciate.

Interposed between the main focusing lens 34 and the anode 22, according to the present invention, are cage deflection means illustrated as a pair of electrostatic deflection plates 46 and 48. These plates are so positioned as to be one on each side of the axis 32, in generally spaced relationship to one another but diverging with respect to the anode 22. While the deflection plates 46 and `48 can be symmetrically lpositioned with respect to the gun axis 32, they need not be. Rather, in a preferred embodiment, the plates 46 and 48 are positioned to maximize the deflection of the electron beam in response to being energized by a predetermined charge which is applied to the deflection plates 46 and 48. The deflection plates 46 and 48 are energized by applying an appropriate voltage potential to the plates 46 and 48.

In a preferred embodiment, a positive voltage potential is applied to only one of the deflection plates, for example plate 46. This Voltage is sufficiently positive to deflect the electron beam a predetermined number of degrees from the beam axis 32. Plate 48 is maintained at the same potential as the anode 22.

When the electron beam is deflected, said beam is directed into a Faraday cage 50.

It is the combination of deflecting means (illustrated by electrostatic deflection plates 46 and 48) with a Faraday cage (illustrated by cage S) which, when added to an electron gun apparatus between a beam generating means and the electron beam scan deflecting means which results in the electron gun apparatus of the present invention Depending upon the particular use to which an electron gun apparatus of this invention is to be placed, the combination of deflection means plus Faraday cage as described herein can be located in an electron gun apparatus at any location therein between final limiting aperture and the electron beam deflection means (e.g. the region in which the beam is scanned). Thus, the combination of Faraday cage and cage deflection means can even be placed between a focusing lens and the electron beam deflection means. However, for descriptive purposes and illustrative purposes herein, the combination of a Faraday cage and deflecting means is located between the primary or main focusing lens means and the final limiting aperture in an electron gun assembly.

It will be appreciated that in some electron gun apparatus known in the art, the means to focus the electron beam may be dynamic instead of static, as described, and hence may be made responsive to appropriate input synchronization signals, as, for example, to accomplish distortion correction of the beam during scanning movements. Dynamic beam focusing means can, of course, be used in electron gun apparatus of this invention without departing from the spirit and scope thereof.

One can employ in place of electrostatic deflection plates 46 and 48 an electromagnetic deflection means, such as an appropriate deflection yoke. While electromagnetic cage deflection means can be used in practicing the present invention, it must be appreciated that the electromagnetic deflection means are inherently slower in deflecting the electron beam relative to an electrostatic electron beam deflection means. This is particularly important when operating at high frequencies associated with horizontal synchronizing signals.

The design of deflection means, especially electrostatic deflection plates, follows the extensive teachings already known to those of ordinary skill in the art for controlling electron beams by means of electric fields. The design of deflection systems, such as one using electrostatic deflection plates, naturally will Vary from one electron gun apparatus to another, depending upon the use intended.

Although Faraday cages are known to those skilled in the art, a preferred form of Faraday cage for use in the present invention is illustrated in FIGURE '2. The preferred Faraday cage is generally a rectangularly-shaped block of carbon, designated by the numeral 60, and has its central region carved out or removed so as to leave a generally rectangularly-shaped cavity 62 formed therein.

Carbon is used as the preferred material of construction for a Faraday cage because carbon produces very little secondary emission when bombarded by an electron beam. Additionally, the cavity 62 within the carbon block 60 is designed to capture substantially all of the electrons `within an intercepted electron beam thereby maximizing the capture of electrons from the beam within the cavity 62. In general, the deeper the cavity v62, the greater the capture of electrons. A suitable lead 64 is conveniently secured to some portion of the carbon block 60. The lead 64 applies the electron beam current from the Faraday cage to an amplifier, metering device or the like.

Although the cavity 62 is not necessary, one primary advantage of using a Faraday cage with a cavity stems from the results obtained. thereby. Thus, for example, if a conducting plate was used in place of a Faraday cage, the secondary emission of the plate will vary over a period of time thereby producing erroneous currents. ln general, a conducting plate Faraday cage tends to increase in secondary emission as the surface of the conducting plate becomes contaminated.

The electron gun apparatus of this invention is suitable for use in a situation where one desires to blank or remove an electron beam from the target area. For example, a typical use is in a television system requiring blanking and lwhich is responsive to a composite video signal including three components: (a) non-composite video signal including an information signal for modulating beam current; (b) synchronization signals for controlling the beam scan on the target area; and (c) blanking signals for removing the electron beam from the target area during a predetermined interval (eg. during the time for return of the beam to the start of a scan path from the end of a preceding scan path). It Will be appreciated that such signals may be in the form of a composite input waveform or they may be in the form of separate input waveforms.

An embodiment of a system for effecting electron beam blanking using the electron gun apparatus of this invention is illustrated by the block diagram of FIGURE 3. In FIGURE 3, elements which are similar to those in FIGURE l are marked with same numerals. The beam blanking system is generally designated as 70.

The electron beam, generated in electron gun apparatus 10, is deflected in a scan pattern on the target 40. A composite video signal on line 72 is applied to a synchronizing signal separator 74. The separator 74 separates the composite video signal into (a) horizontal synchronizing pulses on line 76, (b) vertical synchronizing pulses on line 78 and (c) non-composite video signals on line 80. The non-composite video signal on line 80 is applied via a video amplifier 82, and grid input line 84, to grid 24. The grid 24 and filament cathode 20 are supplied from filament and bias supplies 86 and a high voltage source illustrated as a battery 88 with the positive terminal thereof common to the target 40.

In this embodiment, electrons from the cathode are accelerated by the negative high voltage source 88 electrically connected between the filament and bias supplies 86 and the anode 22. The non-composite video signal applied to grid 24 modulates the electron beam in accordance with the information signals contained within the non-composite video signal.

To accomplish blanking of the electron beam generated in the electron gun apparatus 10, horizontal synchronizing pulses on line 76 and vertical synchronizing pulses on line 78 from the synchronizing signal separator 74 are applied to a blanking generator 90. Concurrently, the horizontal synchronizing pulses are applied via line 76 to a horizontal deflection generator 92 and the vertical synchronizing signals are applied via line 78 to a vertical deflection generator 94. The defiection generators 92 and 94 produce scanning signals which are applied to a horizontal defiection amplifier 96 and a vertical deflection amplifier 98 respectively. The defiection amplifiers 96 and 98 apply scanning signals to the appropriate windings in deflection yoke 44, which yoke is capable of deflecting the electron beam in both a horizontal and vertical direction. The defiection yoke 44 deflects the electron beam in a scan pattern on the target area in response to the generated scanning signals applied thereto.

The blanking generator 90 combines the horizontal synchronizing pulses and the vertical synchronizing pulses applied thereto via lines 76 and 78 respectively to produce a composite blanking signal adapted to cause defiection of the electron beam into the Faraday cage thereby removing said beam from said target 40. The composite blanking signal is applied to a blanking amplifier 100, which amplifier 100 is connected to electrostatic defiection plate 48. Anode 22, aperture disk 28, deflection plate 46, target area 40 and Faraday cage 50 are electrically connected together and to the positive terminal of 'battery 88. Additionally, deflection plate 48 is connected to the positive terminal of battery 88 via a resistor 102.

In operation, when a non-composite video signal associated with a single scan line reaches a blanking interval, the defiection yoke 44 is about to defiect the beam in a new scan line. A blanking signal is produced by blanking generator and applied to blanking amplifier 100. y

beam from the target area 40 during beam retrace. In

this example, the total time duration of a horizontal blanking signal is about 10 microseconds.

The blanking amplifier may have at least one other input 104 in addition to the input from blanking generator 90. Input 104 is capable of applying a control signal to blanking amplifier 100 causing said amplifier to produce the valtage signal necessary to deflect the electron beam from the target area 40- into the Faraday cage 50. Input 104 could receive a control signal deflecting the electron beam from the target area 40 into the Faraday cage 50 at any time. Thus, the blanking signals applied to blanking amplifier 100 could be both the composite blanking signals from blanking generator 90 and other signals applied by additional inputs, for example 104.

In place of defiection plates 46 and 48, one can employ an electromagnetic deflection yoke. For purposes of this invention, electrostatic defiection plates for tlefiecting an electron beam into a Faraday cage are preferred. Additionally, certain applications may permit use of a Faraday cage and a deflection means at other locations than those described herein. In such applications, the beam blanking system 70 could be adapted to blank or extinguish the beam in a manner similar to that of the preferred embodiment.

The electron gun construction of this invention is suitable for use in a situation where one desires to monitor the current level of an electron beam, that is, to measure the electron beam current.

An embodiment of a monitoring system is set forth within FIGURE 4 which illustrates an electron gun apparatus of this invention being used in the generation of an electron beam adapted to scan a target area, for example in a Ivideo recording system. In FIGURE 4, elements similar to those in FIGURES l and 3 are marked with the same numerals.

The beam monitoring system is effective when the electron beam has been defiected in the Faraday cage 50, for example, by means of the beam blanking system 70 illustrated in FIGURE 3 described above. The output signal from cage 50 is applied to a beam current measuring means 110. The magnitude of the electron beam current is dependent upon the design criteria of the electron gun. A triode type electron gun designed in accordance with parameters set forth in the above named reference Electron Optics typically yields target beams having currents in the order of tenths of microamperes to microamperes. For average direct currents, the beam current measuring means may be a direct current microammeter. On the other hand, if the electron beam is pulsed, a calibrated wide bandwidth oscilloscope may be used as the beam current measuring means 110 as those skilled in the art will appreciate.

The electron gun construction of this invention is suitable for use in a situation where one desires to control the current level of the electron beam. A typical use is in a video recording system for recording a composite video signal comprising a non-composite video signal, synchronization signals and blanking signals wherein it is necessary to precisely control the intensity of the electron beam current.

When the electron beam, generated in an electron gun, is defiected into a Faraday cage by the cage deflection means, a current pulse is generated in the cage which is representative of the actual beam current bombarding the target. The current pulse is then used for a control operation. In accordance with the invention, the current pulse is used to control the direct current operating level of the grid-cathode assembly within the electron gun.

Normal operation of the electron beam generating means in this system is such that essentially zero electron beam current is produced at the target area. All non-composite video signals applied to the grid tend to increase the beam current above the zero level.

At the end of a scan line, the electron beam current is essentially zero when the electron beam is deflected into the Faraday cage. The zero current level is established in suitable sample and reference circuits. After the establishment of the zero level in these circuits, they are ready for the reception and -analysis of a reference current pulse from the Faraday cage which is generated by applying a fixed voltage reference pulse to the grid of the electron gun which momentarily turns the electron beam on. Comparison of a set point voltage, corresponding to a desired beam current, with a reference voltage produced from the reference pulse, corresponding to the instantaneous beam current, produces a difference signal if the two voltages are not equal. This difference signal is amplified and applied to the supply circuitry to vary the direct current operating level of the grid. The grid D.C. operating level is changed in a direction to restore the electron beam current to the desired level.

Since this is a sampling system, the correction speed depends upon the design of associated sampling and amplifying circuits. It may be essentially instantaneous, that is, complete correction may occur While the sample pulse is being analyzed, or it may be of a longer time duration particularly if only partial correction occurs while the sample pulse is being analyzed. Exact design of system circuits depends on the application of the complete system as those skilled in the are will appreciate.

The beam control system will be described by referring to the block and partial schematic diagram of FIGURE 4 and to waveforms illustrated in FIGURE 5. In FIG- URE 4, the elements that are similar to those of the electron gun construction of FIGURE l and of the beam blanking system of FIGURE 3 are marked with identical numbers. In FIGURE 5, the waveforms A through F have time as the -abscissa and voltage in percent of maximum as the ordinates.

Referring to FIGURE 4, the servo control system for regulating the electron beam current will now be described. A typical horizontal blanking pulse, illustrated as waveform A from time t in FIGURE 5, is applied to plate -48 from time t1 to time t8 to deflect the electron beam in the Faraday cage 50.

At a later time t2, a horizontal synchronizing pulse is received from the synchronizing signal separator 74 of FIGURE 3 and a typical horizontal synchronizing pulse is shown as waveform B beginning at time t2 in FIGURE 5. The leading edge of the horizontal synchronizing pulse lags the leading edge of the horizontal blanking pulse at a predetermined time interval known as the front porch. In certain applications, the front porch time interval can be substantially zero such that operation of the system will not be affected. The trailing edge of the horizontal synchronizing pulse occurring at a time t3 leads the trailing edge of the horizontal blanking pulse which occurs at time t8 and this time interval is known as the back porch.

The Faraday cage 50 is connected to apply an input to a video amplifier 112. The video amplifier 112 typically may have a bandwidth of about one megacycle and be capable of producing an output voltage in the order of -2 volts. The video amplifier 112 is coupled to a second video amplifier 114 by means of a coupling capacitor 116. The second video amplifier 114 should have a bandwidth similar to that of video amplifier 112 and be capable of producing a reference voltage which is approximately equal to the output from amplifier 112. A keyed clamp 118 is connected by an output line 120 between the input of video amplifier 114 and the capacitor 116. The keyed clamp 118 operates to connect the input of video amplifier 114 to ground thereby establishing a ground reference level.

The keyed clamp 118 comprises four diodes connected in a bridge arrangement, and operation of such a keyed clamp is described in an article entitled Television D.C. Component, by K. R. Wendt, appearing in the RCA Review of March 1948, volume 9, pages 85-111. The keyed clamp 118 is connected to and controlled by a keyed clamp driver 122. The keyed clamp driver 122 receives and is conditioned by the horizontal synchronizing pulses. When the keyed clamp driver 122 receives a horizontal synchronizing pulse, the keyed clamp driver 122 produces keying pulses which conditions key clamp 118 to momentarily connect line 120 and video amplifier 114 to ground. A negative keying pulse is shown as waveform C beginning at time t2 and ending at time t3 in FIGURE 5.

The keyed clamp driver 122 comprises an NPN transistor connected as a phase-splitter amplifier which produces two keying pulses of equal magnitude and opposite polarity in response to each horizontal synchronizing pulse. The keyed clamp driver 122 applies the keying pulses to the keyed clamp 118 by means of capacitors 124 and 126.

Video amplifier 114 has its output connected to a keyed gate 130. Keyed gate 130 is electrically the same as keyed clamp 118 except that a capacitor 132 is interposed between the bridge and ground. The keyed gate 130 is connected to and conditioned by a keyed gate driver 134 via l() capacitors 136 and 138. The keyed gate driver 134 comprises a PNP transistor connected as a phase-splitter amplifier and is functionally the same as keyed clamp driver 122.

A delay line 140 receives the horizontal synchronizing pulses and applies a delayed pulse to a one shot multivibrator 142. A tapped output 144 is connected to delay 140 to apply a partially delayed pulse to a second one shot multivibrator 146. The multivibrator 146 applies a reference pulse, illustrated as waveform D beginning at time t4, to the video amplifier 82. The delay provided by delay 140 for the tapped output 144 is the time interval between times t3 and t4.

The video amplifier 82 applies the reference pulse to the grid 24 to momentarily turn on the electron beam for a period of time illustrated in waveform F from time t4 to t7. When the electron beam is turned on, the Faraday cage 50 samples the electron beam current level.

The multivibrator 142 is connected to the delay line 140 to receive the horizontal pulse and applies an output pulse of a shorter duration to the keyed gate driver 134. The keyed gate driver applies keying pulses via capacitors 136 and 138 to condition the keyed gate 130. A negative keying pulse from the keyed gate driver 134 is illustrated as waveform E and conditions the keyed gate from time t5 to time t6.

A capacitor 132 is connected to the output of the video amplifier 114 and the capacitor is capable of being charged to the output voltage of amplifier 114 during the interval from time t5 to t6 when keyed gate 130 and its driver 134 are actuated. The capacitor 132 is connected to apply the voltage charge appearing thereon to a comparator 148. Comparator 148 has a second input from a set point voltage source 150, which set point voltage is representative of the desired D.C. operating level of grid 24. The comparator 148 produces and applies a difference signal to an amplifier 152 if the two voltages are unequal. Amplifier 152 amplifies and applies the difference signal to a lamp 156, which lamp is capable of varying its intensity in response to the difference signal produced by amplifier 152. A photosensitive resistor 158, capable of varying its resistance in response to the varying illumination from lamp 156, is connected to and conditions the filament and bias supply 86 to vary the D.C. level of the grid 24 as a function of the resistance of the photosensitive resistor 158. The grid 24 receives non-composite video signals from video amplifier 82, and a non-composite video signal of one horizontal scanning line is illustrated as waveform F in FIGURE 5.

In operation, the electron beam is blanked and deflected by the horizontal blanking pulse. Shortly after the beginning of the horizontal blanking pulse, a horizontal synchronizing pulse conditions keyed clamp driver 122 to apply keying pulses to the keyed clamp 118 via capacitors 124 and 126 connecting the input of video amplifier 114 to ground prior to the time the electron beam is momentarily turned on. Concurrently, a horizontal synchronizing pulse is applied to the delay line and a reference pulse is applied as an input to video amplifier 82 resulting in the electron beam being turned on for the duration of the reference pulse which pulse occurs during the back porch time interval. The Faraday cage 50 receives the electron beam and applies an input voltage signal to the video amplifier 112. Video amplifier 112 amplifies the signal and couples it to video amplifier 114 via coupling capacitor 116. The delay line 140 applies the delayed horizontal synchronizing pulse to the multivibrator 142 to produce a keying pulse which conditions the keyed gate driver 134, during the reference pulse time, to sample and charge capacitor 132 to the voltage of the reference voltage output of video amplifier 114. Capacitor 132 applies the reference voltage to comparator 148 which compares the voltage from capacitor 132 to the set point voltage 150. If the two voltages are unequal, the comparator 148 produces a difference signal which is amplified by amplifier 152 and applied to increase or decrease the illumination of lamp 156. The change in illumination of lamp 156 changes the resistance of photosensitive resistor 158 thereby conditioning the filament and bias supply 86 to adjust the D.C. operating level of grid 24 to the desired level.

Having thus described a preferred embodiment of the present invention, it is to be understood that various modifications will be apparent to one having ordinary skill in the art, and all such changes are contemplated as may come within the scope of the appended claims.

What is claimed is:

1. A servo control system for regulating electron beam current generated and directed along a predetermined path by an electron beam generating means at a desired level in an apparatus having a first deflecting means for scanning said electron beam in a pattern on a target, said system comprising,

(a) sampling means including a second deflecting means for collecting said beam and producing a difference signal between said beam current level and said desired level; and

(b) means responsive to said sampling means for varying said beam generating means in response to said difference signal to make said beam current level substantially equal to said desired level.

2. The servo control system of claim 1 wherein said sampling means includes an electron beam collecting means positioned adjacent but outside said path between said first deflecting means and said second deflecting means for intermittently collecting said beam.

3. The servo control system of claim 2 wherein said sampling means includes (a) means operatively coupled to said sampling means for producing a reference signal which is proportional to the level of beam current collected by said sampling means;

(b) means responsive to said reference level producing means for comparing said reference signal to a set point signal, said set point signal corresponding to the desired level of beam current, said comparing means producing a diierence signal the magnitude of which corresponds to any difference between said reference signal and said set point signal; and wherein said varying means is operatively coupled to said comparing means for adjusting the electron beam generating means operation in response to said difference signal.

4. A serve control system for regulating electron beam current at a desired level in an electron gun apparatus adapted to generate, modulate, focus and scan an electron beam in a raster in response to input signals including modulation signals, horizontal synchronizing signals and horizontal blanking signals, said system being operable during horizontal blanking, said system comprising, l

(a) means for generating a reference pulse during horizontal blanking;

(b) means responsive to said generating means for applying said reference pulse to said electron gun apparatus to momentarily turn on the electron beam t produce a sample electron beam current pulse;

(c) means including a sample deflecting means and an electron beam collecting means for deilecting said electron beam and sampling said electron beam current pulse during said reference pulse to produce an output signal;

(d) means operatively coupled to said sampling means for amplifying said output signal to produce a reference voltage;

(e) means responsive to said amplifying means for comparing said reference voltage to a set point voltage, said set point voltage corresponding to a desired level of electron beam current, said comparing means producing a difference voltage if said reference voltage is unequal to said set point voltage; and

(f) means for applying said difference voltage to said electron gun apparatus to make the electron beam current level substantially equal to said desired level.

5. An apparatus for generating and controlling an electron beam having a desired current level, said apparatus comprising:

(a) an electron beam generating means at high negative voltage for producing and directing an electron beam along a path toward a target at low voltage potential;

(b) deflecting means adjacent said path for deflecting a said electron beam from said path;

(c) a sampling means at low voltage for collecting a said defiected beam and producing a difference signal between said deflected beam current level and a said desired current level; and

(d) means responsive to the difference signal of said sampling means for conditioning said beam generating means in response to said difference signal to make said beam curent level substantially equal to said desired level.

6. The apparatus according to claim 5, wherein said sampling means includes:

(a) control means for applying a control signal at a predetermined time to said deecting means to deflect said beam from said path', and

(b) collecting means adjacent said path for collecting said beam defiected thereto by said deflecting means to produce a reference signal corresponding to the current of said electron beam.

7. The apparatus acco-rding to claim 6, wherein said sampling means includes:

(a) means for producing a set point signal corresponding to the desired current of said electron beam; and

(b) comparator means for comparing said reference signal to said set point signal and for producing said difference signal the magnitude of which corresponds to the difference between said reference signal and said set point signal.

8. The apparatus according to claim 7, wherein said collecting means includes (a) means for amplifying said deflected electron beam to produce a reference signal corresponding to the current of said deected electron beam; and

(b) keyed clamp means for momentarily establishing zero current level to said amplifier means.

9. The apparatus according to claim 8, wherein said sampling means includes means for applying said reference signal to said comparator and continuing the application of said reference signal while said control signal is removed from said deection means.

10. The apparatus according to claim 9, wherein said means responsive to the difference signal includes coupling means for operatively connecting the electron beam generating means at high voltage to the comparator means at low voltage.

11. The apparatus according to claim 10, wherein said deecting means includes electrostatic means for deflecting a said electron beam from said path.

12. The apparatus according to claim 10, wherein said collecting means includes a Faraday cage.

13. The apparatus according to claim 10, wherein said coupling means includes (a) a lamp connected to said sampling means to receive said difference signal to afford variation of said lamps illuminating intensity in response to said difference signal; and

(b) a photosensitive means for responding to the intensity level of said lamp to condition said beam generating means to make said beam current level substantially equal to said desired level.

14. The apparatus according to claim 10, wherein said apparatus includes scanning means for causing said beam directed toward a target to scan a pattern on said target in response to input scanning signals.

D 15. The apparatus according to claim 14, wherein (a) said control means is operable during the period of horizontal blanking to deect said electron beam into said collecting means during said blanking pe riod;

(b) means for applying a reference pulse to said beam generating means for producing a beam pulse during said blanking period; and

wherein said comparator means compares the level of said beam pulse to the level of said set point signal for producing a dilerence signal to make said beam current level substantially equal t said desired level.

16. The apparatus according to claim 1S, wherein said keyed clamp means momentarily establishes zero current level to said amplifier means during said sampling time and before said reference pulse is applied to said beam generating means.

17. A method for regulating current of an electron beam at a desired level as said beam is scanned in a scan pattern on a target by a scanning means, said method signal to produce a difference signal if said reference signal is unequal to said set point signal; and

(e) adjusting said electron gun assembly in response to said difference signal to make the electron beam current level substantially equal to said desired level of beam current.

18. The method of claim 17 further comprising the step (f) periodically deflecting said electron beam from said target with said sampling deecting means into an electron beam collecting means between scans of the electron beam on said target to enable said sampling to occur while said beam resides in said electron beam collecting means.

References Cited UNITED STATES PATENTS RODNEY D. BENNETT, IR., Primary Examiner T. H. TUBBESING, Assistant Examiner Us. ci. X,R,

UNITED STATES rATENT OFFICE l l CERTIFICATE OF CORRECTION Patent No. 3,497,762 February 24, 1970 Edward W. Reed, Jr.

It is certified that error appears in the above identified patent and that said Letters Patent are herebycorrected as shown below:

Column l, line 48, after "a" insert beam servo control system suitable for use with such Column 7, line 62, "valtage" should read voltage Column ll line 48 "serve" should read servo Signed and sealed this 13th day of October 1970.

(SEAL) Attest: i

WILLIAM E. SCHUYLER, IR.

Commissioner of Patents Edward M. Fleteher, J r.

Attesting Officer 

